1. Field of the Invention
The present invention relates to a device for boosting a direct current power source voltage to a certain level, and more particularly to such a device mounted on an automotive vehicle for boosting a voltage of an on-board battery.
2. Description of Related Art
A 12-volt battery is usually mounted on an automotive vehicle and is used for various purposes such as starting an engine and supplying power to on-board electrical devices. The voltage of the battery is boosted for supplying a higher voltage to particular devices. An example of such a voltage booster is shown in JP-A-9-74666.
A voltage booster device shown in FIG. 4A attached hereto has been known hitherto. The voltage booster device 110 is composed of: a booster circuit 111 for boosting a direct current power source voltage VB and for outputting an output voltage Vout from an output terminal 121; and a microcomputer 113 for monitoring the output voltage Vout and for supplying voltage-boosting signals according to the monitored output voltage Vout to the booster circuit 111. The booster circuit 111 includes a field effect transistor (FET) M11, a coil L11, a pair of reverse-current-preventing diodes D12, D13, a diode D11, an input capacitor C12, a smoothing capacitor C11, and a resistor R11. These components are connected as shown in FIG. 4A. That is, a drain of the FET M11 is connected to a junction of the coil L11 and the reverse-current-preventing diode D13. A gate of the FET M11 is grounded through the resistor R11, and a source of the FET M11 is grounded.
The microcomputer 113 supplies voltage-boosting signals in a form of pulse-width-modulated signals (PWM) to the gate of the FET M11. The PWM signals are supplied when the output voltage Vout becomes lower than the minimum voltage Vmin at which the voltage-boosting is started, and the supply of the PWM signals is stopped when the output voltage Vout reaches the maximum voltage Vmax at which the voltage-boosting is terminated. In response to the PWM signals, the FET M11 is switched on and off repeatedly. Upon turning on the FET M11, current is supplied to the coil L11, and energy is accumulated in the coil L11. Upon turning off the FET M11, the energy accumulated in the coil L11 is discharged to the output terminal 121 through the diode D13. Thus, the output voltage Vout increases while the PWM signals are present, and decreases when the PWM signals disappear. As a result, the output voltage Vout varies as shown in FIG. 4B.
The voltage booster device 110 constructed as above operates in the following manner. When the power source voltage VB is higher than the minimum voltage Vmin, the power source voltage VB is directly supplied to the output terminal 121 through the diode D11. Therefore, the output voltage Vout is equal to the power source voltage VB. On the other hand, when the output voltage Vout decreases to the level of the minimum voltage Vmin according to decrease in the power source voltage VB, the power source voltage VB is boosted in the manner as described above and the boosted voltage is supplied to the output terminal 121. Thus, the output voltage Vout is kept between Vmin and Vmax.
However, in the conventional voltage booster device 110 described above, the following problems are involved. Since the output voltage Vout varies as shown in FIG. 4B during the voltage-boosting operation, it is highly possible that noises are generated in the booster circuit in accordance with the changes in the output voltage Vout. Further, in case the voltage-boosting signals (PWM signals) are continued to be generated due to failure or trouble in the microcomputer 113, the voltage-boosting continues after the output voltage Vout reaches the maximum voltage Vmax. If the output voltage Vout exceeds a permissible maximum voltage in the voltage booster device 110, the device 110 may be fatally damaged.